In its “I’m here to help you” publication on the home office, the IRS says, “You can use any reasonable method to determine the business percentage” of your home that you use for business.
The publication then lists two commonly used reasonable methods:
Gross square footage. Divide the area (the length multiplied by the width) used for business by the total area of your home.
Number of rooms. If the rooms in your home are all about the same size, you can divide the number of rooms used for business by the total number of rooms in your home.
In your calculations, you should probably consider a third reasonable method, commonly known as the net-square-footage method. ... Log in to view full article.